1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus with the same such as a video camera, a still film/digital camera, a TV camera and a monitoring camera.
2. Related Background Art
Image pickup apparatuses such as video cameras and digital still cameras require, as an image taking optical system, a zoom lens having a wide angle of view, a high zoom ratio and high optical performance.
In general, a high zoom ratio is obtained by increasing a refractive power of a magnification-varying lens unit or increasing a movement amount thereof for zooming. However, such increase of the refractive power and the movement amount of the magnification-varying lens unit increases aberration variation in zooming, which makes it difficult to obtain high optical performance over the entire zoom range.
Therefore, in order to obtain high optical performance of a zoom lens with a wide angle of view over the entire zoom range while realizing a high zoom ratio, it is important to appropriately set a zoom type, a refractive power of each lens unit and a lens configuration.
Zoom lenses are known each of which is constituted by four lens units having positive, negative, positive and positive refractive powers in order from an object side to an image side (image plane side). In such zoom lenses, a zoom lens is also known in which the second lens unit of the four lens units is moved to perform variation of magnification and the fourth lens unit thereof is moved to correct variation of an image plane caused by the variation of magnification and to perform focusing (U.S. Pat. Nos. 5,963,378, 6,166,864 and 7,193,787).
Moreover, as one of the above four-lens-unit zoom lenses, a zoom lens is known which has a high zoom ratio of about 20 times (U.S. Patent Publication No. 2008/0043344 and U.S. Pat. No. 7,466,496).
For these four-lens-unit zoom lenses, it is especially important that a configuration of the second lens unit which is a main lens unit for variation of magnification is appropriately set. For example, inappropriate setting of the refractive power, lens configuration and lens material of the second lens unit makes it difficult to obtain a wide angle of view, a high zoom ratio and high optical performance over the entire zoom range.